This work studies the evolution of hyssop oil composition along the CO2 supercritical extraction curve, under changing extraction conditions, and compares the global composition of the extracts obtained to those from the more usual technique of hydrodistillation (HD). In order to achieve this purpose, the effect on the oil yield and composition of two pairs of pressure and temperature values (90 bar, 40 degrees C and 100 bar, 50 degrees C), three flow rates (0.022 kg/min, 0.017 kg/min and 0.012 kg/min), and three particle sizes of vegetable material (0.8 mm, 0.5 mm and 0.3 mm) are studied. A GC-MS analysis of the oil extracted at different time intervals is carried out. The hydrodistillation of the same vegetable material is also carried out to compare both yield and composition with those from SFE. The major compounds, using both techniques, are 1,8-cineol (75%), terpinen-4-ol (5%), beta-pinene (4%) and pinocarvone (4%) among others. Heavier compounds are found in the supercritical extracts. Finally, the model by Sovova is successfully applied to correlate the experimental data. (C) 2008 Elsevier B.V. All rights reserved.